On October 17, 1961, the Museum of Modern Art (MoMA) in New York accidentally put Henri Matisse’s “Le Bateau” on the wall upside-down. Taking a look at the pics above, you can see why the mistake was made (left picture: right way; right picture: wrong way.).
However, the fact the it was upside-down wasn’t noticed until December 3rd, 47 days later.
*Via On This Day.